The general objective of this research is to determine the effects of centrally acting drugs administered during embryogenesis and the neonatal period on the emergence of behavioral inhibition in the young organism. Specifically, the primary research aim is to determine, by pharmacological intervention, the involvement of central cholinergic and serotonergic pathways in the ontogeny of learned inhibitory stimulus control in the young organism. The development of behavioral inhibition will be determined by comparing the performance of chicks of different ages on standard developmental tests used with mammalian and avian models (e.g., passive avoidance, resistance to extinction), and on experimental paradigms that are now used in conditioned inhibition studies with adult animals (e.g., combined cue test, resistance to reinforcement test, S- generalization tests). Heat reinforcement techniques now permit these specific conditioned inhibition procedures to be used with the young chick. Pharmacological agents will be used to alter synthesis, storage, release or duration of action of putative central nervous system transmitters before and/or drug differences in inhibitory performance are due to changes in sensory, motor, or motivational variables.